Using Scheme records in C code

External modules can create records and access their slots
positionally.

s48_value S48_MAKE_RECORD(s48_value)

(may GC)

int S48_RECORD_P(s48_value)

s48_value S48_RECORD_TYPE(s48_value)

s48_value S48_RECORD_REF(s48_value, long)

void S48_RECORD_SET(s48_value, long, s48_value)

The argument to S48_MAKE_RECORD should be a shared binding
whose value is a record type.
In C the fields of Scheme records are only accessible via offsets,
with the first field having offset zero, the second offset one, and
so forth.
If the order of the fields is changed in the Scheme definition of the
record type the C code must be updated as well.